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ACHEMA 2003
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Microreactor technology offers many benefits for process development and production Millimeter-size reactors, heat exchangers, mixers, pumps, and other tiny devices can offer advantages over their large-scale counterparts.
Multichannel microreactors with channel diameters of the order of 10 to several hundred meters have specific surface areas in the range of 10,000–50,000 m2/ m3, noted Albert Renken at an ACHEMA 2003 symposium on microreaction engineering. "This surface-to-volume ratio is roughly two orders of magnitude higher than that of conventional production vessels," observed Renken, who is a professor of chemical engineering at the Swiss Federal Institute of Technology (EPFL), Lausanne. "As the heat-transfer performance is greatly improved compared with conventional systems, higher reaction temperatures are admissible, leading to reduced reaction volumes and amounts of catalyst. Microstructured reactors are therefore particularly suitable for fast, highly exothermic or endothermic chemical reactions." Flow properties and residence times can be precisely adjusted in small devices, and optimum values for yield and selectivity are achievable, said Wolfgang Ehrfeld, a member of the board of management of Ehrfeld Mikrotechnik, in Wendelsheim, Germany. "The small hold-up and the large specific transport rates result in extremely short response times, providing a favorable basis for efficient process control," he said. "Since microreactors generally can be operated within an extremely wide range of operating conditions, they are powerful tools for process development and the search for novel process routes. And because of their small dimensions, they behave like flame arresters and allow access to controlled reactions in the explosive regime." He added that microreaction systems can, in principle, be applied to the small-scale production of fine chemicals by the parallel operation of a large number of continuously working process items. "The productivity of this numbering-up strategy has been successfully demonstrated by nature for more than a billion years," he said. "Living cells are in effect biochemical microreactors, and multicellular life-forms employ the concept of numbering up in a multitude of variants when they build up organs from many identical cells." Ehrfeld Mikrotechnik markets modular microreaction plants for single-stage and multistage syntheses of chemical and pharmaceutical products. They consist of small, cube-shaped modules that fit together to form controllable plants that can carry out all basic process engineering operations, such as mixing, emulsifying, separating, extracting, and cooling or heating. The chemical reactions occur in fine microchannels. Microreactors typically consist of a number of stacked plates containing microstructures that facilitate improved heat transfer and mixing compared with conventional systems. "Continuously running miniaturized chemical reactors allows higher sample rates and reduced reagent consumption," noted Shari Taghavi, business development manager for Frankfurt-based Cellular Process Chemistry (CPC) Systems, in a lecture at ACHEMA. "These features make microreactors ideal tools for chemical synthesis in the pharmaceutical and fine chemicals businesses. "Removal of the reaction heat, particularly in fast and/or highly exothermic reactions, is a struggle when scaling up," he added. "The possibility of runaway reactions and other hazards always has to be considered when scaling up organic processes from research to production in conventional batch reactors. Reactants in a microreactor, on the other hand, can be added simultaneously, which leads to much shorter residence times and favors reduced by-product generation." The use of parallel microreactor modules provides access to kilogram or even ton quantities, and the use of advanced microreaction technology, such as the CYTOS Lab System exhibited by CPC Systems at ACHEMA, can accelerate the time-to-market for new pharmaceuticals and fine chemicals, Taghavi told C&EN. "Scaling-up bottlenecks are avoided," he said. "The key is numbering up rather than scaling up. A process in a microreactor can be used to produce pilot-plant or even production quantities. Yet no time is needed for transferring a process in a microreactor from research on the lab scale to pilot and production scale." With Clariant's Division of Pigments & Additives, which is also based in Frankfurt, CPC Systems has shown that it is possible to synthesize multiton quantities of an azo-pigment suspension with significantly improved crystal size distribution and colorimetric properties in a microreactor pilot plant. A fundamental understanding of the physics and chemistry underlying microreactor technology and a clear view of its applications have lagged behind the rapid development of chemical processing with microdevices over recent years, according to Volker Hessel, vice director of R&D at the Mainz Institute for Microtechnology (IMM), in Germany. "The situation has been strongly technology dominated, rather than being market driven," Hessel observed. "However, there are signs that this domination is now changing." He pointed out that more than 1,000 publications on applications have been published and industrial sales of IMM microdevices have increased three years in a row by about 300% per year. IMM's catalog includes about 30 off-the-shelf microreactor products. These include a micromixer known as SuperFocus that is able to mix aqueous solutions within 5 milliseconds and has a path length of less than 100 mm and a practical throughput of 10 L per hour. "The catalog also includes a 10-kW gas/gas heat exchanger with 200 parallel plates equipped with flanges for installation in complete production plants," Hessel noted. "This is a nice example of internal numbering-up. The size and throughput of the device paves the way to a completely new category of microchemical processing devices." According to IMM Press Relations Officer Stefan Kurze, external numbering up of several single devices quickly reaches its economic limits because as the number of reactors increases, the amount of control technology must also increase. IMM has recently introduced a flexible liquid distribution system that helps to overcome this problem. The system splits a single stream of liquid into six equal substreams. "The IMM system enormously simplifies process control," Kurze said. "For example, for the distribution of three components to six micromixers, three pumps and three tanks with a total of 18 outlets are required. In contrast, 18 pumps are necessary in a conventional system."
There are two major groups of subcomponents in a typical fluidic system, she noted. "Passive subcomponents--such as channels, mixers, separation structures, connection units, and passive valves--cannot be directly controlled," she explained. "They do not have their own power supply. For example, a passive valve responds to fluidic pressure, which means that it opens or closes automatically if the pressure changes. Active components, on the other hand--such as pumps, active valves, and actuators--can be shut on and off." Bartels Mikrotechnik offers passive fluidic elements to its customers. The company has developed, for example, different types of micropumps made from polycarbonate/polyimide or polyetherimide/polyimide. The pumps use either passive valves or fluidic diodes to control flow. "A fluidic diode has no moving parts," Michelsen noted. "It directs flow by using microstructures with different flow resistance in the two directions." Microreactors suffer from a number of disadvantages. They are, for example, prone to fouling, clogging, and corrosion. "Problems include transport of solids, film formation, surface effects, and the fact that the dynamics of the test plant deviate increasingly from the subsequent production plant" as the degree of miniaturization increases, according to Ralf Böhling, head of the Kinetics Laboratory, BASF, Ludwigshafen, Germany. BASF has used microreactors as process design tools for investigating chemical reactions. "A variety of microreactors are now commercially available for this purpose," he said. "To what extent microreactors could be used for 'microplants'--for example, in industrial research--is being studied. However, it does not currently seem likely that all process steps could be performed using microtechnology." RENKEN NOTED that the main problem in use of microstructured reactors for heterogeneous catalytic reactions is effectively introducing catalytically active microporous materials into the microreactor. "The catalytic materials should have a high activity and selectivity and mechanical stability under reaction conditions," he said. "Therefore, the catalyst layer must be strongly anchored to the reactor wall." He presented a novel concept for microreactor systems involving a tube reactor with a diameter of a few millimeters filled with catalytically active filaments placed parallel to the tube walls. "The arrangement gives flow hydrodynamics similar to multichannel microreactors," Renken said. According to Ehrfeld, the biggest hurdle for microreaction technology for quite some time will probably be lack of operating experience and lack of a wide and scientifically well-founded basis for the layout of microreaction plants. "At present, microreaction technology has not generally established itself in industry, and some of its potential economic benefits are based on hypotheses," he concluded. "Even so, process intensification in chemical engineering will be decisively influenced by it. Microreaction technology opens up new opportunities for cost reduction in process development; in investments in production plants; and in maintenance, safety, transport, and storage. And chemical product manufacturers can transfer research results more quickly into production and therefore react faster to new market trends."
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